PROJECT SUMMARY Streptococcus mutans has the ability to catabolize a wide variety of sugars over a wide range of concentrations, to store carbohydrates in different forms for catabolism during nutrient limitation, and to adapt to the constant changes in carbohydrate source and availability in the oral cavity. Previous iterations of DE12236 established the foundation for the present work using a two-pronged approach that involved in- depth analysis of the regulation of expression of the fruA gene, which encodes an enzyme required for the hydrolysis of homopolymers of fructose and has proven to be an ideal model to study substrate-dependent induction and carbohydrate catabolite repression (CCR). Further, by studying a spectrum of sugar transport and catabolism pathways, it was discovered that the molecular mechanisms in S. mutans for prioritization of sugar utilization by preferred carbohydrate sources (i.e. CCR) deviate substantially from those of paradigm organisms. We posit that the deviation of S. mutans from paradigms for CCR was driven by evolutionary adaptations that have imparted to this organism the necessary degree of flexibility to respond to the wide fluctuations in the amount and type of carbohydrates introduced into the human oral cavity. The present proposal builds on these previous studies by focusing on two intimately intertwined behaviors. The first is a detailed analysis of the molecular basis for persistent memory in the decision network for prioritizing the use of preferred and non-preferred carbohydrate sources. The second focuses on the molecular mechanisms and ecological basis for bistability in the response of populations of S. mutans to the presence of non- preferred carbohydrates, such that under certain conditions only a sub-population of the cells in a community produces the gene products needed for catabolism of non-preferred carbohydrates. Importantly, the sub- population that activates the genes liberates substantial quantities of hexose into the environment, which can be used by ?cheaters? that do not activate the catabolic systems for the non-preferred carbohydrate. The fact that abundant members of the microbiome engage in these behaviors has profound implications for the dynamics of development, persistence and pathogenicity of oral biofilms. To understand the molecular basis for, and ecological consequences of, these behaviors, we present two Specific Aims: Aim 1. Analyze the genetic basis for persistent memory in carbohydrate utilization and its contribution to fitness. Aim 2. Dissect the molecular basis and benefits of ?cheating? behaviors in the catabolism of disaccharides in sub-populations of S. mutans.